Agriculture Reference
In-Depth Information
From the point of view of population genetics, a useful definition of a population
is 'a group of interbreeding individuals that exist together in time and space'
(Hedrick, 2004). This definition comprises both gene flow, as interbreeding causes
allele frequencies to be more or less uniform throughout a population, and natural
selection, in that any one area at any one time will have particular features which
affect the relative fitnesses of different genotypes. For sexual species, definitions of
a population based on interbreeding and on natural selection overlap to a large
extent, because individuals must be present in the same area at the same time in
order to interbreed, and are therefore likely to be subject to similar selection
pressures. However, many plant pathogenic fungi have a prolonged asexual phase.
For instance,
P. striiformis
has no known sexual stage (Hovmøller
et al.
, 2002;
Enjalbert
et al.
, 2005) so while individual fungi which coexist in time and space -
perhaps even in a single field - cannot interbreed with one another, they are
nonetheless subject to the same selective forces. In practical terms, therefore, they
are members of a single population, though not one in which interbreeding occurs.
(b) Changes in populations
More than in most other organisms, the nature of a population of a plant pathogen
may change radically through a year. There are two especially striking ways in
which this can happen. In the first case, there may be distinct populations of a
pathogen on different varieties of a crop, but these may merge to form a single
population on another crop. In the 1970s and 1980s, many spring barley varieties in
the UK had gene-for-gene resistances to mildew whereas winter barley varieties
generally had either no gene-for-gene resistance at all or only ineffective resistances.
The mildew fungus,
B. graminis
f.sp.
hordei
, therefore existed as several distinct
populations on spring cultivars, in that colonies on one variety produced conidia
which were not necessarily able to infect other varieties. However, almost all clones
could successfully infect winter barley cultivars, on which there was therefore
essentially a single population (Wolfe, 1984). Such a process of successive division
and amalgamation of populations has two consequences for the pathogen's
population genetics. Firstly, negative linkage disequilibrium between virulence
genes required to overcome the resistances of different spring barley varieties may
arise, because different pathogen clones carry those virulences (Wolfe and Knott,
1982; Østergård and Hovmøller, 1991). Secondly, pathogen virulence genes may
become recombined through sexual reproduction on winter barley plants, giving rise
to clones able to overcome a previously effective combination of resistances (Brown
et al.
, 1993).
In the second case, there may be a great difference between the mobility of the
asexual and sexual phases of a pathogen, so that there are two different types of
population. In its asexual phase, the septoria tritici blotch fungus,
M. graminicola
(anamorph
Septoria tritici
), is dispersed over very short distances, as splash-borne
pycnidiospores. Consequently, there is no correlation between the genotypes of
single-pycnidium isolates of
M. graminicola
even between sampling sites just 10 m
apart (McDonald
et al.
, 1995). Hence, as a result of low mobility within a cropping
